1. Field of the Invention
The invention relates to a system for controlling the timing of ignition in an engine. More particularly, the invention relates to a system for changing the timing of an engine so as to heat an exhaust system connected to the engine.
2. Description of Related Art
Vehicles, such as snowmobiles, conventionally include an engine such as an internal combustion engine in order to enable them to move under their own power. In particular, two cycle engines are used in a variety of vehicles because of their high power to weight ratio, simplicity, etc.
It is known to design the exhaust system for such a vehicle so that it is “tuned”, such that the harmonic characteristics of the exhaust system allow for increased power and fuel efficiency, and reduced engine emissions.
Conventional tuned exhaust systems have limitations. For example, the harmonic characteristics of an exhaust system depend in part on the temperature of the exhaust system. Thus, an exhaust system normally can be fully tuned only for a narrow range of temperatures. Conventionally, an exhaust system is tuned for what is expected to be the typical sustained operating temperature for a particular vehicle.
However, when the engine in a conventional vehicle is started, the temperature of the exhaust system typically does not begin at the normal operating temperature. If the exhaust system is significantly colder than the normal operating temperature, for which it has been tuned, the exhaust system will be out of tune.
Thus, an engine that is started cold does not receive the benefits of a tuned exhaust system. Consequently, the power and fuel efficiency of the engine may be reduced until the exhaust system warms, and the engine emissions likewise may be increased.
Furthermore, even if an engine has been started, and has been allowed to run for a significant period of time while the vehicle is stationary, the engine heat generated may not be sufficient to heat the exhaust system to its normal operating temperature. In practice, exhaust systems in conventional vehicles do not reach normal operating temperature until the vehicle has been moving for some period of time; idling or revving the engine without moving the vehicle often is not sufficient. Thus, even if the engine is running, the exhaust system may remain out of tune until the vehicle has traveled a significant distance.
The limitations of conventional systems with regard to exhaust tuning are of particular importance in conditions where a vehicle must start from a standstill, and achieve high speeds in a short time, for example when racing.
Likewise, the limitations of conventional systems may be especially pronounced in cold conditions, such as those under which snowmobiles commonly are used, since at colder ambient temperatures the difference between the actual temperature of the exhaust system and the tuned temperature may be significantly greater.
A brief description of the operation of a conventional engine may be helpful in understanding the present invention.
FIG. 1 shows a conventional two-cycle engine 10, as known from the prior art. As shown, the engine 10 includes a crank case 13 and at least one cylinder 12 with a cylinder wall 26 and a cylinder head 14. A piston 16 is movably disposed within the cylinder 12. The engine 10 also defines an intake port 30 that allows an ingoing mixture 38 to enter the engine 10, a transfer port 31 that allows the incoming mixture 36 to move from the crank case 13 to the cylinder 12, and an exhaust port 32 that allows an outgoing mixture 38 to exit the engine 10.
The piston 16 and a crank web 20 are connected with a connecting rod 18 such that the connecting rod 18 pivots where it attaches to both the piston 16 and the crank web 20. Thus, as the piston 16 moves up and down in the cylinder 12, the crank web 20 is made to turn about its axis of rotation 22. Typically, a crank shaft (not shown) is connected to the crank web 20 at the axis of rotation 22, the crank shaft carrying the power to the vehicle's drive system.
FIG. 1A shows the engine 10 with the piston 16 in its uppermost position, also referred to as “top dead center”. For purposes of the following description, top dead center will also be considered to be 0 degrees with respect to a circular path traveled by the end of the connecting rod.
In the top dead center position, both the transfer port 31 and the exhaust port 32 of the engine 10 are blocked by the piston 16. Matter cannot enter or exit the cylinder 12 through either port.
From top dead center, the piston 16 moves downward as shown in FIG. 1B. In the position shown, the engine is 90 degrees after top dead center. The exhaust port 32 is unobstructed in this position, and the outgoing mixture 38 exits the cylinder 12 therethrough. Conventionally, the outgoing mixture 38 for a two-cycle engine includes the combustion products from the engine's fuel and oil, and oxygen-depleted air. The outgoing mixture moves from the exhaust port 32 toward the exhaust system (not shown).
The piston 16 continues to move downward as shown in FIG. 1C. In the position shown, the engine is 180 degrees after top dead center. This position also may be considered to be 180 degrees before top dead center, and is sometimes referred to as “bottom dead center”. The exhaust port 32 is still unobstructed in this position, and the outgoing mixture 38 may continue to exit the cylinder 12 therethrough. In addition, the transfer port 31 is now unobstructed, allowing an incoming mixture 36 to pass therethrough from inside the crank case 13. Conventionally, the incoming mixture 36 for a two-cycle engine includes fuel, oil, and air.
The piston 16 then moves upward as shown in FIG. 1D. In the position shown, the engine is 90 degrees before top dead center. The exhaust port 32 is still unobstructed in this position, and the outgoing mixture 38 may continue to exit the cylinder 12 therethrough. However, the transfer port 31 is now obstructed, so no more incoming mixture 36 may enter the cylinder 12 therethrough. In addition, at this point an intake valve 33 opens at the intake port 30, allowing the incoming mixture 36 to be drawn into the crank case 13.
Conventionally, at some point before top dead center, the fuel and air in the cylinder 12 are ignited by the igniter 24. As illustrated in FIG. 1E, the igniter 24 includes a spark plug that produces a spark 34.
In the position shown in FIG. 1E, both the exhaust port 32 and the transfer port 31 are obstructed by the piston 16, and matter may not pass through either port. In addition, the reed valve 33 commonly is closed at this point, preventing any more of the incoming mixture 36 from being drawn into the crank case 13. When the cylinder 12 ignites, fuel combusting within the cylinder 12 generates pressure that drives the piston 16 downward again, repeating the cycle from FIG. 1A.
Thus, as shown in FIG. 1E, the position at which ignition conventionally occurs, referred to herein as the operating ignition position, occurs before the engine 10 reaches top dead center. As illustrated, the position is 15 degrees ahead of top dead center. The engine angle of the operating ignition position may vary somewhat depending upon the particular design of the engine 10. Likewise, the engine angle of the operating ignition position may vary somewhat during operation depending on conditions such as engine speed. However, conventionally ignition occurs significantly ahead of top dead center.